1. Field of the Invention
The present invention relates to current sources, and in particular, to current mirror circuits.
2. Description of the Related Art
Current mirrors are often used, particularly in monolithic integrated circuits, to provide constant current sources. With such circuits, the output current I.sub.O will ideally be proportional (e.g. equal) to and track the input, or reference, current I.sub.R, with such proportionality being maintained consistently over a wide range of reference current I.sub.R magnitudes. However, conventional current mirror circuits experience a number of problems in trying to maintain this proportionality of the output current I.sub.O to the reference current I.sub.R. To varying degrees, the ability of conventional current mirrors to produce an output current I.sub.O that tracks the reference current I.sub.R is dependent upon the betas (.beta.), i.e. base-to-collector current gains, of the transistors and the circuit current gain. The dependency upon the transistors' current gains is particularly problematic over a wide range of reference current I.sub.R magnitudes since the transistors' betas tend to vary.
Referring to FIG. 1, a conventional current mirror circuit includes two PNP transistors (FIG. 1A) or two NPN transistors (FIG. 1B) connected as shown. It can be shown that the current transfer characteristic for this type of current mirror circuit can be expressed as follows: ##EQU1## where: N=circuit current gain
B=transistor base-to-collector current gain (".beta.") PA1 B.sub.M =transistor base-to-collector current gain (".beta.") of transistor Q.sub.MP (or Q.sub.MN) PA1 M.di-elect cons.{1,2,3,4} PA1 B=B.sub.1 =B.sub.2 =B.sub.4
Referring to FIG. 2, another conventional current mirror circuit, commonly referred to as a cascode output current mirror, includes three PNP transistors (FIG. 2A) or three NPN transistors (FIG. 2B), and a diode, connected as shown. As can be shown, the current transfer characteristic for this circuit can be expressed as follows: ##EQU2## where: Circuit current gain (N)=1
Referring to FIG. 3, another conventional current mirror circuit, commonly referred to as a "super diode" current mirror, includes three PNP transistors (FIG. 3A) or three NPN transistors (FIG. 3B) connected as shown. As can be shown, the current transfer characteristic for this circuit can be expressed as follows: ##EQU3##
Referring to FIG. 4, another conventional current mirror circuit, commonly referred to as a cascode output, "super diode" current mirror, includes four PNP transistors (FIG. 4A) or four NPN transistors (FIG. 4B) connected as shown. As can be shown, the current transfer characteristic for this circuit can be expressed as follows: ##EQU4## where: Circuit current gain (N)=1
Referring to FIG. 5, another conventional current mirror circuit, commonly referred to as a "Wilson" current mirror, includes three PNP transistors (FIG. 5A) or three NPN transistors (FIG. 5B) connected as shown. As can be shown, the current transfer characteristic for this circuit, which typically has a current gain of unity, can be expressed as follows: ##EQU5##
Referring to FIG. 6, another conventional current mirror circuit, commonly referred to as a modified "Wilson" current mirror, includes six PNP transistors (FIG. 6A) or six NPN transistors (FIG. 6B) connected as shown. While this circuit offers high output impedance (due to its cascode output), and some compensation for variations among the current gains (.beta.s) of the transistors, it nonetheless suffers from the same errors caused by significant reductions in the transistors' current gains (.beta.s) at high collector current levels. (Further discussion concerning this circuit can be found in J. G. Holt, Jr., "A Two-Quadrant Analog Multiplier Integrated Circuit", 1973 IEEE International Solid-State Circuits Conference Digest of Technical Papers, at page 181, the disclosure of which is incorporated herein by reference.)
Further discussion of some of these and other conventional current mirror circuits can be found in U.S. Pat. No. 4,528,496, the disclosure of which is incorporated herein by reference.